Thermocouple



March 15, 1932. E. HERZOG ETAL THERMCOUPLE Filedo April 20. 1927 EL il 0 2110 400 60D BOD |000 lZGD |400 \60D |800 Invent-,ove Eugene Herzog, Harrold Lwatson,

Their Attorney. f

Patented Mar. 15, 1932 UGENL HERZOG AND HAROLD L. WATSON, oF LYNN, MASSACHUSETTS, -AssIeNoRs 'ro GENERAL ELECTRIC c 'oMPANinI A coR-PoHA'rIoN or NEW YORK -THERMOCOUPLE I' Application med April 2o, 1927.l serial. No. 185,324.

The present invention relates to thermof couples, and comprises particularly thermocouples adapted accurately. to measure very high temperatures for prolongedgperiods of 5 time.

measuring devices it has been possible only by delicate laboratory methods to accurately measure temperatures as high as 14000 C.

1U ThermoelectricV pyrometers when employed 15 or gases which are usually given oft' by materials when heated to high temperatures.

lVe have discovered that a thermoelectric couple comprising tungsten andgraphite has thermoelectric and other physical properties Heretofore in the use of high temperaturev 20 peculiarly adapted for the determination of such high temperatures. Thermocouples em-Y bodying the invention have proved to be adapted for continuously indicating and ac-Av curately measuring high temperatures be- 2`5 tween about 1000 to 2300o C. for prolonged periods of time. One of the novel features Aof our invention comprises the combination with such a couple of means for protecting the couple from the effect of deleterious gases.

3U Other novel features which we believe to be characteristic of our invention are set'forth in the appended claims.

The invention itself, however, will best be understood from the following specification when considered in connection with the accompanying drawings 1n which Fig. .l 1s a vertical cross-section of our improved form of thermocouple; Fig. 2 is a cross-sectional view taken on the line 2-2 of Fig: 1,; Fig. '3

is a modified form of our invention; and Fig.- 4 1s a curve whlch shows the voltage/ differ-J ence between the elements of the a temperature range.

In carrying the invention 1nto effect, we

couple for employ a thermocouple consisting of graphite and tungsten members which are given -a suitable heat treatment prior to assembly,

in order to provide a thermocouple the elements of which will be stabilized or in other words will have substantially constant electrical characteristics. The heattreatment comprises heating the tungsten member, for

example to a temperature of about 2000"' C.

and aging it at that temperature for approximately two hours; The heating and aging of the tungsten `produce a' structure having substantially constantelectrical characteristics. It is probable that this process results in the production of large tungsten crystals, thereby preventing a slow crystal growth of the tungsten member when subjected to heat which would cause variations-in the'electrical 'characteristics of the tungsten and, therefore, in readings obtained on an indicating device connected to the thermocouple. The 'graph- -ite member is heated and agedin the same manner and at the same temperature asthe tungsten, the heating andagin'g ofthe graphite preventing substantially anyA subsequent' graphitization of carbon in the Agraphite member. l

The thermocouple, when assembled,'is protected-by means of a'layer of suitable'carbon, such as charcoal, which'sur'rounds that. part of the couple exposed to high temperatures.

Ajportion ofthe charcoal combines at high v the lowerend of the graphite tube 3 andprovides, with rod 1, the hot junction of the thermocouple. 1 A

The upper end of the thermocouple remote from the hot junctioniscooled andmaintained at a desired temperature by means of a'water jacket consisting of'an inner metallic member 5 in contact with, the graphite tube and an outer metallic member 6. The lower and tion the hot junction of thexthermocouple as y to accommodate rod.1, engages and closes upper ends of the jacket are closed byv plugs 7 and 8, respectively. Water is supplied to the jacket through a pipe 9, the kougl'tlet being indicated at 10 (see Fig. 2). To provide for the Iproper circulation of the cooling water in the jacket 2, verticalpr longitudinal partitions 11 and 12 are inserted between members 5 and 6 at opposite sides of rthe water jacket.

A tapered metal cap 13 is mounted on the upper end of the water jacket and is provided with a closure member or plug 14 .of conducting material mounted in an insulating bushing 15. The plug 14 is provided at its lower end with screw threads adapted to engage a conducting rod 16. This rod is provided with an adjustable plate 17I and iswelded at at its lower end to the upper end of the tungsten rod 1, as indicated at 18. An insulating bushing 19 is mounted withinthe tube 5 to surround the welded joint between the tungsten member and the rod 16 and carries at its upper end an l insulating washer 19. A spring 20 bears against the washer 19 and plate 17 and serves to maintain the graphite plug 4 in good electrical contact with the graphite sleeve 3,- and the tungsten rod -l'in good contact with plug 4.

A sheath 21, the character of which' de; pends on the conditions under which the couple is to be used, surrounds-the lower end of the thermocouple. The sheathis provided with a screw threaded ring 22 adapted to engage a second ring 23 mounted on an annular member24 secured to the lowerv end of the water jacket. Movement of ring 23 adjusts the position of the sheath 21 with respect to the thermocouple. The sheath is provided with pins 25 movable in openings in the member 24, and is substantially filled with-suitable carbon, such as finely divided charcoal, as indicated at 2 6, which completely surrounds tlie lower portion of the thermocouple. If' desired, a graphite cap 29 may be lapplied at the lower end of the thermocouple to prevent the couple from stickingr to the sheath 21.

Electrical connections may be made from the thermocouple to any suitable indicating device by means of a metal screw threaded binding post 27 'electrically connected to the vgra'phiteelement of the thermocouple through the water jacket, and by means of a screw threaded member 28 provided in plug 14.

When the lower end of the thermocouple is placed in a furnace,l the temperature of which is to be measured, the heat fromthe furnace'causes-a portionof the vcharcoal to unite withwhatever oxygen may be present in the sheath, and vform carbon monoxide. This gas 'forms alayercompletely surrounding the lower` lportion of the thermocouple, and since`the gas thus formed is inactive with respect to the elements of the thermocouple, it eii'ectively prevents any deleterious action on the 'thermocouple'dueto the high temperaslowly tures employed.` The charcoal may be renewed from time to time asfound necessary.

Sheath 21 may be made of various materials and its composition depends upon the circumstances under which it is intended to employ the thermocouple. In a gaseous oxidizing flame, it will beifound advantageous to employ a carborundum sheath, while if the temperature of molten iron is to be measured by immersion, thisv sheath will usually be composed of graphite bonded with clay.

In the modified forml` of the invention in-l couple. If desired, an inert or inactive gas may be producedfby'dropping wood alcohol into the hot zone. The particular gas employed, however, will, of course, depend upon the temperatures to be measured.

I carrying this form of the invention into effect, the ring 23 is provided with a supply pipe 30 which may be connected to a suitable source of gas supply, such as an ordinary commercial tank containing, for example, hydrogen.

from the tank to the space between the sheath 21 and thermocouple. To provide for the circulation of the gas, a small opening 31 is made in the lower end of the graphite member 3. The heated gas enters opening 31, passes upwardly through members 3 and 19 and-then through a small openingV 32 in the insulator-.15 and out into the atmosphere. l

Before starting operation, it may be found desirable-to flush out sheath 21 and remove any air which may -have accumulated therein. To promote such action, an outlet pipe vThe gas may be supplied very,

33 having a valve 34 is provided inthe ring 23. This arrangement allows a large volume of gas to be passed through the sheath, thereby effectively removing any accumulated air. Valve 34 is then-closed, and a quantity of gas,

only suicient to ensure exclusion of air,.per mitted to How through'the sheath. 4An asbestos gasket 35 renders sheath 21 substantiallyy gas-tight.

. A thermocouple consisting of tungsten and graphite L-members and constructed in the manner above disclosed is particularly advantageous. since it will givea relatively high and substantially constant electromotive force for any given temperature and will operate at temperatures up to 2300 C., and even higher temperatures, depending on the conditions of use.

A` thermocouple consisting of the above elements has a further advantage due to the lowl temperature at which reversal of its electromotive force takes place. In the. operation of thermocouples, there is a tempera.-

ture at whichthe electromotive force passes through zero and reverses. This reversal takes place 'with many couplesaat relatively high or extremely low temperatures. With the present type of thermocouple, this reversal takes place at about 100 C.,'so that the couple is not aected by this action throughout any portion of its operating range.

This particular feature of the invention is clearly illustrated in Fig. 4, in which the absciss represent temperatures in degrees centigrade,\ and the ordinates millivolts. Curve A was actually determined in practice, while the lowei` dotted portion B represents points which were calculated but not observed, and indicate the direction which the curve A would normally be expected to take. The reversal of electromotive` force of the couple takes place at a point slightl above C., thereby providing an unusua ly lon working range for the thermocouple. It wil be noted `from the curve shown in Fig. 4; that if the cold junction of the couple were varied in temperature from zero to C., the error introduced in the temperature determination would be quite small; in fact,

the maximum error introduced would be approximately 15 C., which obviously woild be negligible in high temperature determinations.

What we claim as new, and desire to secure by LettersPatentof the nited States, is:

.1. The method of treating the elements of a thermocouple consisting of arcarbon member and a tungstenv member, which comprises heating said elements-prior to assembly at a.

temperature of about 2000 C. for about two hours to thereby produce substantially constant electrical characteristics in said elements. l f

2. The method of treating the elements f a thermocouple'which -consist of tungsten and carbon, which comprises aging the elements prior to assembly at a temperature of about 2000 C. to thereby produce substantially constant electrical characteristics in said elements.

VIn witness whereof, wehave'hereunto set our hands this 16th dayof April, 1927, f EUGENE MRZOG.

. HAROLD L. wATsoN. 

